1. Field of the Invention
The present invention generally pertains to methods for the sensing of, and sensors for the detection of, very low frequency pressure waves, particularly in the atmosphere as infrasound.
The present invention particularly concerns fiber optic pressure sensors, and the arrangement of fiber optic pressure sensors as an interferometer in order to detect infrasound with common mode rejection of (i) temperature and (ii) strain or vibration noise, and with a high signal-to-noise ratio of (iii) detected infrasound versus wind noise.
2. Background of the Invention
2.1 Infrasound
Infrasound signals are very low frequency (0.01 to 10 Hz) pressure waves that travel through the atmosphere. They have been detected using conventional pressure gauges; usually electro-mechanical barometers which measure pressure at a point.
Noise from wind makes sensing these signals difficult. Indeed, the key weakness in existing infrasound detection systems is the rapid deterioration in the signal-to-noise ratio with increasing wind velocity. To increase sensitivity researchers have attached conventional pressure gauges to long tubes perforated with variously distributed holes in attempts to average out the noise from the wind. Propagation effects in these mechanical filters limit their effectiveness.
Studies in infrasound detection often center on techniques of noise reduction. Almost all work to date consists or recording pressure at a single point while an attempt is made to force the pressure at that point to be representative of the pressure averaged along a line or over an area. As previously stated, a series of perforated pipes or permeable hoses are typically connected to a microphonic sensor. Pressure noise along the pipe""s length is partially incoherent while, for pipe lengths less than the wavelength of interest (typically a few hundred meters), the signal is coherent. The aim of the mechanical filter is to add pressure variations at discrete inlets along its length so that the incoherent noise will average away while the signal is enhanced. Conceptually, one can envision the ideal case or many sensors in an array separately recorded and their signals added together electronically.
Prior art observations of infrasonic noise versus wind speed through and in a system or about 0.5 Hz bandwidth will be shown in graph form in FIG. 1 of this specification. Reference Clauter and Blandford, 1996. A prior art observed spectrum of infrasonic noise under calm wind conditions at Warrarmunga, Central Australia from Christie, et al., 1978, will be shown in FIG. 2 of this specification.
Significant efforts have gone into the designs of prior art mechanical filters. One of the more venerable devices is the Daniels filter (Daniels, 1959), which relics on pipes of varying diameter to create a series or acoustic impedance changes with the hope of reducing acoustic reflections in the pipe. Burridge (1971) analyzed this type of xe2x80x9cpipe-microbarographxe2x80x9d and similar configurations. When added acoustically in the filter pipe, there exists a phase delay for each clement caused by the finite speed of sound. Burridge modeled pipes with varying (i) dimensions, (ii) numbers of inlets, and (iii) acoustic impedances to find the best compromise response flattening and attenuation minimization. In all cases, however, the response clearly is a compromise. The difficulty becomes greater as the frequency increases. As Burridge showed, flat response above 0.1 or 1 Hz are not attainable.
2.2 Infrasound Detection as Part of the Comprehensive Test Ban Treaty (CTBT)
As part of the international monitoring system of the Comprehensive Test Ban Treaty (CTBT), infrasound signals in the band 0.02 to 4 Hz must be detected in the presence of ambient noise generated chiefly by wind. Thus effectiveness of acoustic filters employed in standard infrasound sensors is limited by pressure propagation and attenuation characteristics within the filter. To improve the filtering characteristics, an optical fiber for sensing the integrated pressure variations along a line has been designed. The optic fiber sensor can easily average over kilometer-scale lengths of arbitrary geometry with an averaging bandwidth governed by the speed of light and thus should offer significant practical advantages in reducing the effect of wind noise and thus increasing the signal-to-noise ratio over a wide bandwidth.
2.2.1 Specific Previous Infrasound Detection
Infrasonic monitoring is an effective, low cost technology for detecting atmospheric explosions of nuclear weapons. The low frequency components of explosion signals propagate to long ranges (a few thousand kilometers) where they can be detected with arrays of infrasound sensors.
A prototype infrasound system for use under a comprehensive test ban treaty has been constructed by the United States. The system is near real time, automated and unattended.
The United States Infrasound Sensor System Prototype is consistent with a specification in a Preparatory Commission document prepared under the Comprehensive Test Ban Treaty (CTBT), The system is a four-element array in a triangular layout with an infrasound sensor element at each corner and one in the center. The prototype infrasound sensor element spacing is 1 km, however, the specifications provide for an optional spacing up to 3 km. All prototype components are exportable and operable over a wide range of environmental conditions.
System security is provided by housing the array element hardware (sensor, digitizer, authenticator, etc.) in a secure enclosure. The enclosure is buried in the ground and is protected by active (switch closure) and passive tamper detection devices.
The objectives of the United States Infrasound Sensor System Prototype are to reliably acquire and transmit near-real-time infrasonic data to facilitate the rapid location and identification of atmospheric events. The prototype system is also directed to providing documentation that could be used by the United States and foreign countries to procure infrasound systems commercially to fulfill their CTBT responsibilities.
Detail requirements for infrasound monitoring set by The Conference on Disarmament and the CTBT Preparatory Commission (PrepCom) are as follows.
A wideband microbarograph, or equivalent such as the instrument of the present invention, should exhibit a flat frequency response from 0.02 to 4.0 Hz.
An array of four such elements, with a sensor spacing from 1 to 3 km is typical. The Conference on Disarmament and the CTBT Preparatory Commission (PrepCom) recommended an equilateral triangular array, 1 to 3 km on a side. An array element is located at each corner and at the center.
Sensor noise should be at least 18 dB below the minimum acoustic noise of 5.0 mPa at 1.0 Hz.
Sensors should include acoustic filtering of wind noise. In previous sensors this is realized with noise reduction pipes.
Resolution should be better than 1 count per mPa.
Dynamic range should be at least 108 dB.
The sensor array would usefully provide a data stream at a sample rate of about 10 samples per second (sps). Data from all array elements would desirably be authenticated.
An exemplary prior art microbaragraph (above) is a 10xe2x80x3 diameter Chaparral Physics model 4.11. The vendor re-packaged the sensor to accommodate the above data survey features. The infrasound system includes four array elements, intra-site communications, and a host receiving station.
The components that make up the infrasound system also include an array of infrasound detector elements each containing a sensor, a digitizer with GPS, and a data authenticator. A host receiving station contains a multiplexer, data displays and state-of-health displays. The host receiving station also transmits the data to the NDC in near real time.
The array element hardware (sensor, digitizer, authenticator, etc.) is housed in an enclosure (a utility box) to provide system security. The enclosure is buried in the ground and is protected by tamper detection devices such as a switch closure. This level of protection is necessary to protect the data authentication process.
2.2.2 Specific Previous Detection of Infrasound, and Ongoing Research, Under the Comprehensive Test Ban Treaty (CTBT)
The infrasound monitoring envisioned for the International Monitoring System (IMS) under the CTBT uses a worldwide network of infrasonic sensors to monitor the low-frequency acoustic signals resulting from explosions. This is a primary IMS system for monitoring atmospheric explosions, and it should also be a resource for shallow buried underground and underwater events. To monitor the CTBT, arrays of infrasound microphones will be employed that are capable of detection of kiloton-type explosions out to 3,000 to 5,000 km.
The goal of the Department of Energy""s Infrasound Monitoring Research is to improve the US government""s capability to detect and identify low-frequency acoustic signals from atmospheric, shallow buried, or moderately shielded explosions. This invention is in accordance with this goal.
The DOE Infrasound research priorities are a follows:
The signal-to-noise ratio would be desirably reduced through enhanced array design and optimized noise-reduction methods. The present invention will be seen to accomplish this in a broad and substantial manner.
Understanding of the propagation of infrasound waves by developing advanced tools and compiling global wind data would desirably be improved.
An enhanced understanding of natural infrasound sources, such as meteors, is desired so as to reduce false alarms.
A fully tested and documented prototype infrasound system that reliably acquires infrasound data for the rapid location and identification of atmospheric events should be made commercially available. The Infrasound System is such a commercializable, fully documented, system.
Optimal functioning of the IMS infrasound network would desirably be ensured by the assistance of its manufacturer in the development of site-survey and station installation procedures.
DOE Laboratories are involved in infrasound research, circa 1999. The primary DOE laboratory involved in infrasonic monitoring research is Los Alamos National Laboratory. LANL is supported by research efforts in the private sector and by Sandia National Laboratories for developing, testing and documenting the prototype Infrasound system and evaluating infrasound sensors.
2.3 Properties of Optical Fibers
In another technical area, optical fibers are known to be sensitive to hydrostatic pressure. They thus present some potential as sensors of acoustic pressure, including infrasound. Alas, optical fibers are also sensitive to temperature and to strain.
In order to make an infrasound sensor using optical fiber sensor, it would seemingly be necessary to make an optic fiber that was appropriately sensitive to pressure variations caused by infrasound while being insensitive to temperature and strain, the two other parameters to which optical fibers are largely sensitive. The optical fiber sensor would desirably be of almost unlimited length, thereby permitting the averaging of wind-induced noise over long distances.
The present invention contemplates improving the signal-to-noise ratio between wind noise and infrasound during the detection of infrasound by using a new type pressure, or sound, or infrasound detector, namely an optic fiber, that extends for so many hundreds and thousands of meters that wind noise is not coherent along its length. Pressure changes in the optic fiber are detectable as variations in a modulated optical (light) signal transmitted along the length of the fiber. The wind noise is integrated and detected as, at most, a small offset signal to the detected infrasound signal.
The present invention further contemplates combining, in the manner of a Michelson or Mach-Zehnder or equivalent interferometer, two of the new type sensors to make an infrasound detection system. Namely, (i) a first greatly-linearly-extending fiber optic pressure sensor that is exposed to the atmosphere is combined with (ii) a second, co-parallel, fiber optic pressure sensor that is hermetically sealed. These two sensorsxe2x80x94plus such external components as permit of insertion and detection of the optical (light) signal within each optic fiber, and the transformation of the optical (light) signals so detected into electrical signals subsequently interpretable as infrasoundxe2x80x94collectively constitute a sensitive infrasound detection system with a high signal to noise ratio. This is because the arrangement of two fiber optic pressure sensors as an interferometer accords common mode rejection of (i) temperature and of (ii) strain or vibration noise, including such as may be due to earth movement.
1. A Fiber Optic Infrasound Sensor
Therefore, in one of its aspects the present invention is embodied in a fiber optic pressure sensor.
The fiber optic pressure sensor is characterized in that a fiber opticxe2x80x94in which optic fiber pressure is sensed by (i) injection of optical radiation into the fiber and (ii) detection of the change in optical length of the fiber proportional to change in the integral of pressure along the fiberxe2x80x94extends in the atmosphere for greater than the coherence length of wind in the atmosphere. Regardless of what other pressures within the atmosphere and in the environment of its deployment in the atmosphere the fiber optic may sense, the fiber optic will be less sensitive to pressure changes due to wind. This is because any atmospheric pressure changes arising from wind not being coherent over the extent of the optic fiber, will be substantially integrated along the length of the fiber, and this integration over greater than a coherence length of a function being integratedxe2x80x94i.e., the function of wind pressure along the length of the optic fiberxe2x80x94will produce a greatly reduced signal.
The fiber optic pressure sensor preferably extends substantially linearly in the atmosphere, in a substantially straight line. It preferably so extends for more than one hundred meters, more preferably for more than one kilometer.
Pressure within the fiber optic is preferably sensed by (i) injection of laser light optical radiation into the fiber.
Importantly the diameter of the fiber permits detection of atmospheric infrasound from at least 0.4 to 10 Hz frequency. It should be understood that the incremental step size of detectable variation in the propagation path along the fiber is in the order of fiber diameter. Therefore an appropriately small diameter permits of the detection of atmospheric infrasound. The diameter of the optic fiber is preferably less than 200 xcexcm, and is more preferably about 125 xcexcm.
2. A Fiber Optic Infrasound Detection System
In another of its aspects the present invention is embodied in a fiber optic infrasound detection system.
The system includes a source of modulated light illumination and an equal-arm Michelson or Mach-Zehnder or equivalent interferometer. The interferometer has a first optic fiber exposed to atmosphere and extending for greater than 100 meters from the source of modulated light illumination, and a second optic fiber, hermetically sealed from the atmosphere, extending alongside the first optic fiber from the source of modulated light illumination.
Light from the source of modulated light illumination is transmitted along both fibers. It is so transmitted in accordance with any changes in optical length of the fibers responsively to, inter alia, pressures and temperatures and strains to which each fiber is subject.
It will in particular be recognized by those familiar with light transmission along optic fibers that, for pressure-induced changes in optical length:
xcex94lop/lopxcex1∫xcex94P(x)dx
In words (as opposed to mathematics): the change in optical length relative to optical length is proportional to the integral of the changes in pressure along the length of the optic fiber.
A detector receives the light transmitted along both optic fibers and detects changes in the difference between the optical lengths of both optic fibers. The detector is sensitive in a frequency band including from at least 0.2 to 4 Hz.
According to this construction, changes in optical lengths as result from, inter alia, (i) changes in temperature and (ii) changes in strainxe2x80x94including as are due to earth movementxe2x80x94are all substantially canceled due to the interferometric arrangement of the two optic fibers. However, (iii) atmospheric pressure changes arising both from wind and from infrasound can sensed in the frequency band. But, it should be recalled, pressure changes due to the wind are not coherent over the greater than 100 meter extent of the optic fibers. Accordingly, detection of changes in optical length due to atmospheric pressure changes arising from the wind is substantially the integration of these wind-induced changes along the length of the fiber, and is small.
Thus the detector serves to sense atmospheric pressure changes due to infrasound over the lengths of both optic fibers at a high signal to noise ratio over those atmospheric pressure changes due to any of (i) temperature, (ii) strain and vibration, and (iii) wind.
In the most preferred construction, the source of modulated light illumination and the detector are both at the same end of the optic fibers. Namely, both the first and the second optic fiber extends from the source of modulated light illumination to mirrored ends. Light is thus reflected along both optic fibers. (The reflected light path is in accordance with, inter alia, the pressures and temperatures and strains to which each fiber is subject.)
The source of modulated light illumination is typically a laser, and is more typically a laser diode.
The first and the second optic fiber normally extend linearly for up to a kilometer or more.
The second fiber of the equal-arm Michelson or Mach-Zehnder or equivalent interferometer is preferably hermetically sealed in a metal tube, more preferably in a 0.030xe2x80x3 diameter stainless steel tube. To ensure that the (i) thermal and (ii) strain characteristics of both fibers are as nearly equal as is possible, the first fiber may also be placed within a like metal tube save only that the tube is perforated, and open to the atmosphere.
The detector of the light reflected in both fibers preferably includes a beamsplitter of (i) light from the source of modulated light illumination and (ii) the light reflected in both fibers; a signal processor receiving a signal from the beamsplitter; and a modulator responsive to the signal processor for modulating light from the source of modulated light in accordance with the signal received by the signal processor. By this feedback pressure changes within band are tracked.
3. A Method of Detecting Infrasound
In yet another of its aspects the present invention is embodied in a method of detecting infrasound.
In the method modulated light illumination is provided by, and from, a light source into each of a first optic fiber exposed to atmosphere, and a co-parallel second optic fiber hermetically sealed from the atmosphere. Each fiber extends from the source of modulated light illumination to a mirrored end.
Light is reflected along both fibers in accordance with inter alia, pressures and temperatures and strains to which each fiber is subject. The light reflected in both fibers is detected in a frequency band including from at least 0.2 to 4 Hz so that changes in optical lengths of both fibers as are due to, inter alia, variations in (i) temperature, (ii) strainxe2x80x94including as is due to earth movementxe2x80x94and (iii) pressurexe2x80x94as arises both from wind and from infrasoundxe2x80x94will all be detected.
However, detected changes in optical length due to variations in (i) temperature and in (ii) strain are substantially canceled due to the interferometric arrangement of the two fibers.
Moreover, changes in optical length due to wind-induced pressure changes are not coherent over the extent of the fiber. Thus the detection of pressure changes arising from the wind is a substantial integration of these changes along the length of the fiber, and is small.
Accordingly, the detecting primarily serves to sense atmospheric infrasound, only.
The providing of modulated light illumination is preferably from a laser light source, and more preferably from a laser diode.
The co-parallel extensions of the first optic fiber that is exposed to the atmosphere, and the second optic fiber that is hermetically sealed, is preferably linear (in a straight line), and for at least 100 meters. The extension is more preferably linearly for at least 1000 meters.
The detecting preferably includes (1) splitting in a beamsplitter of (i) light from the source of modulated light illumination and (ii) the light reflected in both fibers; (2) receiving in a signal processor a signal from the beamsplitter, and (3) processing this received signal to cancel signal contributions from changes (i) in temperature and (ii) in strain (including as are due to earth movement), while sensing (iii) pressure as arises both from wind and from infrasound.
After this (2) receiving and processing in the signal processor, the detecting further preferably includes modulating in a modulator responsively to the signal processor light from the source of modulated light in accordance with the signal received by the signal processor.
These, and other aspects and attributes of the present invention, will become increasingly clear upon reference to the following drawings and attached specification.